1. Field of the Invention
This invention relates to a catalytic combustor for a combustion turbine and, more specifically, to a device for suppressing vibration in the plurality of cooling tubes which pass through the fuel/air mixture plenum within a catalytic combustor.
2. Background Information
Combustion turbines, generally, have three main assemblies: a compressor assembly, a combustor assembly, and a turbine assembly. In operation, the compressor compresses ambient air. The compressed air flows into the combustor assembly where it is mixed with a fuel. The fuel and compressed air mixture is ignited creating a heated working gas. The heated working gas is expanded through the turbine assembly. The turbine assembly includes a plurality of stationary vanes and rotating blades. The rotating blades are coupled to a central shaft. The expansion of the working gas through the turbine section forces the blades, and therefore the shaft, to rotate. The shaft may be connected to a generator.
Typically, the combustor assembly creates a working gas at a temperature between 2,500 to 2,900 degrees Fahrenheit (1371 to 1593 degrees centigrade). At high temperatures, particularly above about 1,500 degrees centigrade, the oxygen and nitrogen within the working gas combine to form the pollutants NO and NO2, collectively known as NOx. The formation rate of NOx increases exponentially with flame temperature. Thus, for a given engine working gas temperature, the minimum NOx will be created by the combustor assembly when the flame is at a uniform temperature, that is, there are no hot spots in the combustor assembly. This is accomplished by premixing all of the fuel with all of the of air available for combustion (referred to as low NOx lean-premix combustion) so that the flame temperature within the combustor assembly is uniform and the NOx production is reduced.
Lean pre-mixed flames are generally less stabile than non-well-mixed flames, as the high temperature/fuel rich regions of non-well-mixed flames add to a flame""s stability. One method of stabilizing lean premixed flames is to react some of the fuel/air mixture in conjunction with a catalyst prior to the combustion zone. To utilize the catalyst, a fuel/air mixture is passed over a catalyst material, or catalyst bed, causing a pre-reaction of a portion of the mixture and creating radicals which aid in stabilizing combustion at a downstream location within the combustor assembly.
Prior art catalytic combustors completely mix the fuel and the air prior to the catalyst. This provides a fuel lean mixture to the catalyst. However, with a fuel lean mixture, typical catalyst materials are not active at compressor discharge temperatures. As such, a preburner is required to heat the air prior to the catalyst adding cost and complexity to the design as well as generating NOx emissions, See e.g., U.S. Pat. No. 5,826,429. It is, therefore, desirable to have a combustor assembly that burns a fuel lean mixture, so that NOx is reduced, but passes a fuel rich mixture through the catalyst bed so that a preburner is not required. The preburner can be eliminated because the fuel rich mixture contains sufficient mixture strength, without being preheated, to activate the catalyst and create the necessary radicals to maintain a steady flame, when subjected to compressor discharge temperatures. As shown in U.S. patent application Ser. No. 09-670,035, which is incorporated by reference, this is accomplished by splitting the flow of compressed air through the combustor. One flow stream is mixed with fuel, as a fuel rich mixture, and passed over the catalyst bed. The other flow stream may be used to cool the catalyst bed.
One disadvantage of using a catalyst is that the catalyst is subject to degradation when exposed to high temperatures. High temperatures may be created by the reaction between the catalyst and the fuel, pre-ignition within the catalyst bed, and/or flashback ignition from the downstream combustion zone extending into the catalyst bed. Prior art catalyst beds included tubes. These tubes were susceptible to vibration because they were cantilevered, being connected to a tube sheet at their upstream ends. The inner surface of the tubes were free of the catalyst material and allowed a portion of the compressed air to pass, unreacted, through the tubes. The fuel/air mixture passed over the tubes, and reacted with, the catalyst. Then, the compressed air and the fuel/air mixture were combined. The compressed air absorbed heat created by the reaction of the fuel with the catalyst and/or any ignition or flashback within the catalyst bed. See U.S. patent application Ser. No. 09-670,035. 
The disadvantage of such systems is susceptibility of the tubular configuration to vibration damage resulting from: (1) flow of cooling air inside of the tubes, (2) flow of the fuel/air mixture passing over the tubes transverse and longitudinal to the tube bundle, and (3) other system/engine vibrations. Such vibration has caused problems in the power generation field, including degradation of the joint (e.g. braze) connecting the tubes to the tubesheet and degradation of the tubes themselves, both resulting from tube to tube and/or tube to support structure impacting.
There is, therefore, a need for a dampening device for a catalytic reactor assembly of a combustion turbine, which suppresses vibration of the plurality of closely oriented parallel tubes.
There is further a need for a dampening device for a catalytic reactor assembly to effectively baffle and promote even distribution of the fuel/air mixture.
There is further a need for a dampening device for a catalytic reactor assembly that provides a stronger, reinforced attachment of the tubes to the tubesheet.
There is further a need for a dampening device for a catalytic reactor assembly that provides resistance to reverse flow of the fuel/air mixture caused by eddie currents, which in turn can lead to backflash (premature ignition of the fuel in the combustor).
There is further a need for a dampening device for a catalytic reactor assembly that maintains appropriate pressure differential to promote uniform distribution of the fuel/air mixture and ensure adequate cooling is maintained.
The present invention satisfies these needs, and others, by providing a dampening device with expanded regions on the tubes that maintain tube to tube contact and thus suppress vibration. The invention consists of at least one expanded region and at least one narrow region on each tube. The expanded region may be achieved by a localized increase in the nominal tube circumference, a sleeve or furrel placed over the tube and enlarging the circumference, or by machining or swaging the tube to create narrow regions. The localized expansions extend for a portion of the tube length, having a gradual transition between the nominal circumference and the center of expansion. If the tube is cut or swaged to create narrow regions in between the nominal tube circumference regions, the nominal tube circumference would serve as the expanded region. There may also be multiple expanded regions on a tube.
The expanded regions may be symmetric along the tube length and/or around the tube circumference. Alternatively, the expansions could be non-symmetric, or even single-sided. Expansions located at the ends of the tubes are examples of single-sided expansions. Moreover, an expanded region on one tube may contact another expanded region on another tube, or alternatively, may be staggered so that an expanded region on one tube contacts the narrow region of an adjacent tube. The tubes and the expanded regions thereon could be a variety of shapes such as bulges, ridges, and/or helices, so long as the flow path around the tubes and desired pressure drop is maintained.
By maintaining tube to tube contact, adjacent tubes support one another rather than impact one another during various modes of vibration. Moreover, expansion of the tubes to provide contact at a plane just downstream of the fuel/air inlet has been predicted analytically to effectively baffle and to promote even distribution of the fuel/air mixture.
The upstream ends of the tubes may be bulged or expanded to provide additional support of the fragile joints (e.g. brazes) where the tubes attach to the tube sheet. Similarly, the tubes may be bulged at their downstream ends to provide resistance to reverse flow and therefore backflash, because eddie currents are eliminated by the gradual bulging profile. The expanded or flared inlet and outlet ends of the tubes also provide a substantial reduction (e.g. approximately 14 percent for a flared inlet, 22 percent for a flared outlet) in pressure differential between the air inside the tubes and the air/fuel mixture passing over them. Avoiding an excessive pressure differential allows more effective cooling.